class Solution {
public:

    int dx[4] = { 0,0,1,-1 };
    int dy[4] = { 1,-1,0,0 };
    int maxDistance(vector<vector<int>>& grid)
    {
        int m = grid.size();
        int n = grid[0].size();
        int Max = -1;
        queue<pair<int, int>> q;
        vector<vector<int>> dist(m, vector<int>(n, -1));
        for (int i = 0; i < m; i++)
        {
            for (int j = 0; j < n; j++)
            {
                if (grid[i][j] == 1)
                {
                    q.push({ i,j });
                    dist[i][j] = 0;
                }
            }
        }

        while (q.size())
        {
            auto [a, b] = q.front();
            q.pop();
            for (int k = 0; k < 4; k++)
            {
                int x = dx[k] + a, y = dy[k] + b;
                if (x >= 0 && x < m && y >= 0 && y < n && dist[x][y] == -1)
                {
                    dist[x][y] = dist[a][b] + 1;
                    Max = max(Max, dist[x][y]);
                    q.push({ x,y });
                }
            }
        }
        return Max;
    }
};